The development of low thermal expansion cordierite ceramic honeycombs for applications such as filters and catalyst supports for treating the exhaust gases emitted by trucks and automobiles has now proceeded continuously for close to thirty years. The most widely used honeycombs of this type can be characterized as highly microcracked and highly oriented bodies, in that the cordierite crystals present in the honeycombs microcrack on cooling after firing and are preferentially aligned as the result of the honeycomb extrusion forming process. Both of these features combine to significantly reduce the thermal expansion coefficient and improve the resistance to thermal shock damage of the formed honeycombs.
However, as engine emission requirements are tightened, further improvements in the mechanical and thermal properties of filters and catalyst supports will be needed. For example, the mechanical strengths of current honeycomb products are somewhat limited, due to the highly microcracked nature of the cordierite phases making up the materials. Strength improvements will be required as new honeycomb substrates and filters designed with thinner honeycomb walls and/or substantially higher wall porosities are developed.
The presence of microcracks can also be problematic in other ways. For example, commercial washcoating materials of high-surface-area alumina used to support emissions control catalysts on these honeycombs can readily penetrate the microcracks present in the cordierite, in some cases resulting in unacceptable increases in the coefficient of thermal expansion (CTE) and elastic modulus of the coated products. Further, soot and ash particulates present in combustion engine exhaust streams can also penetrate the microcracked structure and can similarly impact thermal expansion and elastic properties. The most objectionable result of these properties changes is that the thermal shock resistance of the honeycombs, i.e. their resistance to breakage when exposed to rapid temperature changes, can be degraded.